Among the technologies being considered, developed or currently deployed for use in treating abnormalities of human and animal tissue is focused ultrasound (FU). Focused ultrasound devices use ultrasound transducers to deliver generally thermal or cavitational dose to a small, well-defined spot at some fixed or focal distance from the transducer surface. Typically, the region to be treated is larger than the small spot of dose that is delivered from the transducer.
One way to deliver thermal dose to a larger region is to move the transducer so that the small spot of thermal dose is scanned over the region that is to receive thermal or cavitational dose. Another way is to move the patient relative to the transducer. The latter approach is often used in extracorporeal devices where the transducer is located outside the patient. Such is the case with the EXABLATE system. The former approach is used often in devices where the transducer is located inside the patient. Such is the case with devices such as the Sonatherm and SONABLATE devices.
In devices where the transducer is introduced into the patient and is moved potentially relative to the patient, it is typically deployed in a probe. Typically, such probes include an acoustic window for passage of the FU and a way to coupling the acoustic window to the tissue to be treated. Coupling involves providing a continuous acoustic path between the transducer and the tissue being treated. The coupling mechanism, typically degassed water, is contained by an acoustically invisible membrane made typically of materials such as latex. The acoustic window, through which the FU will pass, and its coupling mechanism are brought in contact with the tissue through which the FU will pass. The coupling mechanism also is used as a spacing device in order to position the focal post of the transducer the correct distance from tissue. By increasing the volume of coupling fluid contained within the membrane, the tissue can be pushed further from the transducer. Finally, the fluid used in the coupling mechanism can be circulated around the transducer in order to remove heat buildup from around the transducer or even to increase the temperature of the transducer. Cooling can have a protective effect on tissue against which the probe is brought, removing excess heat delivered to tissue directly adjacent to the probe if that tissue is not to be part of the region being treatment (heated). Conversely, heated fluid can be used to increase the amount of heat retained by the tissue if it is to be included in the planned treatment.
In instances where the probe is introduced into a sterile environment, the portion of the probe that comes in contact with tissue should be sterile. The water used to provide the coupling mechanism also should be sterile. There also are instances where the probe is introduced into a nonsterile environment, such as the rectum for transrectal ablation of the prostate, and where contamination of the water path, and therefore anything that comes in contact with the water used in the water path, is possible.
Ultrasound probes also are used for imaging purposes. In such an application, an ultrasound transducer mounted inside a probe is surrounded by a tissue coupling medium in order to provide an acoustic path to the tissue being imaged. This tissue coupling medium may be internal to the probe housing in direct contact with the transducer and/or may be applied to the surface of the probe in the form of a gel or contact medium.